A linear fixed shaft drive assembly for use in a self-stacking industrial/commercial-grade spiral/gyro oven or freezer that includes a fixed, non-rotatable shaft having a cylindrical mounting portion, a bearing cup supported by the mounting portion, and a hub and shaft base that includes a shaft portion and flange portion. The flange portion is configured to couple to a first side of a flange extending from the bearing cup. A sprocket is mounted to an opposite second side of the flange extending from the bearing cup. The sprocket includes teeth configured to engage with a ball chain assembly of the oven or freezer. The bearing cup includes at least two permanently lubricated bearings. Rotation of the shaft portion of the hub and shaft base causes the bearing cup and sprocket to rotate on the mounting portion of the fixed, non-rotatable shaft. Also a method for retrofitting an oven or freezer having an existing shaft assembly mounted within a thermal chamber of the oven or freezer, and a method for servicing an oven or freezer having an existing linear fixed shaft drive assembly.